A device of this class has been known from West German Patent No. DE-PS 24,28,598. In this prior-art device, a folding chute consisting of two bars, is used for forming folds. The bars are parallel to one another and one end of them projects freely in the direction of the sewing direction. A loop is laid around the two bars and is tucked and is pulled into the folding chute to form a double fold. The loop can be pulled into the folding chute to varying depths, so that the depth of fold can be varied. The bars can be designed as thin but stable wires or thin, narrow, strip-shaped spatulas. These bars are arranged, together with a stop that determines the width of the loop and the workpiece clamp transporting the double fold to a sewing machine for edge sewing by the stitch formation site, on a carriage that is movable relative to the needle. In addition, the bars can be removed from the area of the workpiece in order to facilitate insertion and removal of the workpiece. The prior-art device only makes it possible to form a heart-shaped double fold, which is not laid out flat, from a single tucked loop.
A device for forming a plurality of folds simultaneously has been known from West German Offenlegungsschrift No. 34,05,721. This device is designed as a workpiece clamping holder for sewing devices for sewing the edge zones of lining material and top fabric in the tip corner zone and side corner zones of a necktie. This device is equipped with three clamping plates hinged to each other in order to receive the lining material and the top fabric in a clamping manner in conjunction with a tensioning device between the clamping plates, wherein the side of the middle plate facing the top fabric is designed with a central folding knife and lateral webs for forming a middle fold and two side folds to receive an extra width of top fabric. On the upper plate clamping the top fabric, the workpiece clamping holder is provided with folding slides for folding over and holding down the side folds formed at the side corners. This, together with the clamping plates, clamps the fabrics to be sewn together in the edge zones.
Folding edges are provided on holders connected elastically to the upper clamping plate. During the lowering of the upper clamping plate following the formation of the middle fold, the folding edges come into contact with the top fabric on both sides of the webs. Furthermore, the folding edges provided between the tip corner zone and the side corner zones are designed such that the upper clamping plate following formation of the middle fold prior to deposition of the other folding edges. Because the folding processes take place one after another, it is achieved that the fabric or the extra width of material is pulled in to the folds from defined areas in the fabric cut. Additional, pneumatically operated pushers are provided to lay the side folds flat after they have been formed.
This prior-art device is intended only for forming single folds on workpieces of uniform size, in which the folds are always formed in the same area.
Finally, West German Patent No. 26,29,117 should be mentioned in connection with the technological background of the present invention. That patent discloses a folding device which is arranged in the area of the stitch formation site of a sewing machine and has a plurality of folding plates arranged at spaced locations from each other. The folding plates cooperate in pairs with a fork-shaped folding slide each, for forming a multiple fold each. A plurality of the folding plated are formed one after another, e.g., at the edge of curtains, wherein the fork-shaped folding slides are displaced in opposite directions. Each multiple fold consists of two single folds arranged in a mirror-inverted pattern and at laterally spaced locations, and the single folds are covered by a common middle fold. The multiple fold is fixed in this folded state by needles which are movable in height and are pulled out of the multiple fold after a pressing belt, transporting the zone of the multiple fold through the sewing station, has been lowered onto the multiple fold.
Since the two single fold pairs are formed simultaneously and the fabric length needed for fold formation can be pulled in only from one side of a multiple fold already sewn, the great number of deflecting edges on the folding plates and the folding slides leads to a relatively high friction resistance This friction resistance causes considerable stress not only for the material to be folded and the folding tools, but for the driving device for the folding tools as well. This must be taken into account by designing the folding tools and the driving device as a function of these stresses, and it makes the entire folding device more expensive.